On Tuesday, Google will switch on a tweak to its algorithm that will give preference to 'mobile-friendly' sites when users perform a search on a mobile device.
The change, which has colloquially become known as 'Mobilegeddon', will arrive tomorrow and threatens to bury websites that haven't been optimised to display correctly on various sizes of mobile devices, while also boosting the ranking of mobile-friendly sites.
Google announced the change in February and outlined what it would consider "mobile friendliness" when deciding on where to rank websites in search results from April 21. It warned at the time that the change would have a "significant impact" on search rankings - especially given that around half of searches are thought to be conducted from a mobile device.
Those wanting to see if their websites are likely to be affected can use Google's mobile friendly test tool, available here.
Both Target and Amazon earlier this year reported that 60 percent of traffic to their respective sites during peak shopping seasons came from mobile devices, highlighting the importance for online retailers of supporting mobile device displays.
According to Search Engine Land, Google is believed to get about half its search traffic from mobile devices. One of the company's top search execs said in 2014 that he expected the number of mobile searches to top desktop queries that year, a phenomenon likely to have been aided by emerging markets such as India where mobile usage is far higher than PC usage.
Despite Google's warning nearly two months ago, it seems many website operators are unprepared for the change. The Financial Times reported today that fashion brand Versace, Microsoft's own Windows Phone site, and some financial services websites could suffer as a result of the changes to Google's algorithm. The change won't affect searches from desktops or tablets, the FT added.
Last year Google rolled out a gentler initiative, which saw it adding "mobile-friendly" labels to websites that were optimised for mobile.
Google is now making changes to mobile search as it faces tougher competition from Facebook for mobile ad revenue.
Last week the company introduced a new feature that will return apps when people search using Android devices. The links to the apps will take the user back to the Google Play Store.